Gallium Nitride Devices

ABSTRACT

Semiconductor structures comprising a III-nitride (e.g., gallium nitride) material region and methods associated with such structures are provided. In some embodiments, the structures include an electrically conductive material (e.g., gold) separated from certain other region(s) of the structure (e.g., a silicon substrate) by a barrier material in order to limit, or prevent, undesirable reactions between the electrically conductive material and the other component(s) which can impair device performance. In certain embodiments, the electrically conductive material may be formed in a via. For example, the via can extend from a topside of the device to a backside so that the electrically conductive material connects a topside contact to a backside contact. The structures described herein may form the basis of a number of semiconductor devices including transistors (e.g., FET), Schottky diodes, light-emitting diodes and laser diodes, amongst others.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/634,332, filed Dec. 4, 2006, which claims priority to U.S.Provisional Patent Application Ser. No. 60/741,609, filed Dec. 2, 2005,which are incorporated herein by reference.

FIELD OF INVENTION

The invention relates generally to gallium nitride material devices and,more particularly, to gallium nitride material devices includingconductive regions, as well as devices and methods associated with thesame.

BACKGROUND OF INVENTION

Gallium nitride materials include gallium nitride (GaN) and its alloyssuch as aluminum gallium nitride (AlGaN), indium gallium nitride(InGaN), and aluminum indium gallium nitride (AlInGaN). These materialsare semiconductor compounds that have a relatively wide, direct bandgapwhich permits highly energetic electronic transitions to occur. Suchelectronic transitions can result in gallium nitride materials having anumber of attractive properties including the ability to efficientlyemit blue light, the ability to transmit signals at high frequency, andothers. Accordingly, gallium nitride materials are being widelyinvestigated in many microelectronic applications such as transistors,field emitters, and optoelectronic devices.

SUMMARY OF INVENTION

Gallium nitride material structures are provided, as well as devices andmethods associated with such structures.

In one aspect, a gallium nitride material semiconductor device structureis provided. The device structure comprises a substrate; a galliumnitride material region formed on the substrate; a first contact formedon the gallium nitride material region; an electrically conductivematerial layer formed over, at least a portion of, the substrate; and abarrier material layer separating the electrically conductive materiallayer from the substrate.

In another aspect, a gallium nitride material semiconductor devicestructure is provided. The device structure comprises a substrateincluding a top surface and a back surface; a gallium nitride materialregion formed on the front surface of the substrate; a first metalregion formed on the gallium nitride material region; a second metalregion formed on the back surface of the substrate; a barrier materialformed on, at least a portion, of a sidewall of a via extending throughthe gallium nitride material region and the substrate; and anelectrically conductive material formed on the barrier material in thevia extending from the first metal region to the second metal region.

In another aspect, a gallium nitride material semiconductor devicestructure is provided. The device structure comprises a semiconductorstructure comprising a substrate including a top surface and a backsurface; a gallium nitride material region formed on the top surface ofthe substrate; a source electrode formed on the gallium nitride materialregion; a gate electrode formed on the gallium nitride material region;a drain electrode formed on the gallium nitride material region; apathway of electrically conductive material extending from the sourceelectrode to a conductive region formed on a back surface of thesubstrate.

In another aspect, a gallium nitride material semiconductor devicestructure is provided. The device structure comprises a siliconsubstrate including a top surface and a back surface; a gallium nitridematerial region formed over the top surface of the silicon substrate; acomponent bonded to the back surface of the substrate with a eutecticcomprising an electrically conductive material and silicon, wherein avia extends from the back surface of the silicon substrate.

In another aspect, a method of forming a semiconductor device structureis provided. The method comprises forming a gallium nitride materialregion on a front surface of a substrate; forming a first metal regionon the gallium nitride material region; forming a second metal region ona back surface of the substrate; forming a via extending through thegallium nitride material region and the substrate; forming a barriermaterial on, at least a portion of, a sidewall of the via; and formingan electrically conductive material the barrier material in the viaextending from the first metal region to the second metal region.

In another aspect, a method of forming a semiconductor device structureis provided. The method comprises forming a gallium nitride materiallayer on a silicon substrate; forming a via that extends through atleast a portion of the silicon substrate; forming a layer comprising anelectrically conductive material on a back surface of the siliconsubstrate to form a semiconductor structure; and heating the structureto form a liquid eutectic comprising the electrically conductivematerial and silicon; and cooling the liquid eutectic to form a bondbetween the semiconductor structure and a component.

Other aspects, embodiments and features of the invention will becomeapparent from the following detailed description of the invention whenconsidered in conjunction with the accompanying drawings. Theaccompanying figures are schematic and are not intended to be drawn toscale. In the figures, each identical, or substantially similarcomponent that is illustrated in various figures is represented by asingle numeral or notation. For purposes of clarity, not every componentis labeled in every figure. Nor is every component of each embodiment ofthe invention shown where illustration is not necessary to allow thoseof ordinary skill in the art to understand the invention. All patentapplications and patents incorporated herein by reference areincorporated by reference in their entirety. In case of conflict, thepresent specification, including definitions, will control.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 illustrates a gallium nitride material-based device structureaccording to an embodiment of the present invention.

FIG. 2 illustrates a semiconductor device according to an embodiment ofthe present invention.

DETAILED DESCRIPTION

The present invention relates to semiconductor device structurescomprising a III-nitride (e.g., gallium nitride) material region andmethods associated with such structures. In some embodiments, thestructures include an electrically conductive material (e.g., gold)separated from certain other region(s) of the structure (e.g., a siliconsubstrate) by a barrier material in order to limit, or prevent,undesirable reactions between the electrically conductive material andthe other component(s) which can impair device performance. For example,the barrier material may limit, or prevent, undesirable reactionsbetween a gold layer and a silicon substrate. In certain embodiments,the electrically conductive material may be formed in a via. Forexample, the via can extend from a topside of the device to a backsideso that the electrically conductive material connects a topside contactto a backside contact. The structures described herein may form thebasis of a number of semiconductor devices including transistors (e.g.,FET), Schottky diodes, light-emitting diodes and laser diodes, amongstothers.

FIG. 1 illustrates a semiconductor structure 10 including a galliumnitride material region 18 formed over a substrate 14 according to oneembodiment of the invention. In this embodiment, the structure includesan electrically conductive material layer 24 formed in a via 20. Theelectrically conductive material layer provides a conductive pathwayassociated with the structure. For example, the electrically conductivematerial layer can connect a contact 28 on a topside 29 of the structureto a contact 30 on a backside 31, as shown. A barrier material layer 22is formed on sidewalls 26 of the via to separate the electricallyconductive material from the substrate thereby limiting undesirablereactions between the electrically conductive material and the substratewhich may otherwise occur, for example, at elevated temperatures insubsequent processes. As described further below, the structure includesan arrangement of layers between substrate 14 (e.g., a siliconsubstrate) and the gallium nitride material region 18 which may promoteformation of high quality gallium nitride material. The arrangement inthis embodiment includes a strain-absorbing layer 12, an intermediatelayer 15, and a transition layer 16.

When a layer is referred to as being “on”, “over” or “overlying” anotherfeature (e.g., layer or substrate), it can be directly on the feature,or an intervening layer also may be present. A layer that is “directlyon” another layer or substrate means that no intervening layer ispresent. It should also be understood that when a layer is referred toas being “on”, “over” or “overlying” another feature (e.g., layer orsubstrate), it may cover the entire feature, or a portion of thefeature.

It should be understood that not all of the features (e.g., layers)shown in FIG. 1 are present in all embodiments of the invention and thatthe illustrated features may be otherwise positioned within thestructure. Also, additional features may be present in otherembodiments. Additional embodiments are shown in the other figuresand/or described further below.

Electrically conductive material layer 24 may formed of any materialhaving suitable electrical conductivity. In certain embodiments, it maybe preferable for the electrically conductive material layer to comprisegold. When comprised of gold, the electrically conductive material layermay also comprise other elements such as metals (e.g., copper). Theother element(s) may be alloyed with gold to form a layer having asingle composition. Also, the other element(s) may be present in aseparate layer formed on top of a gold layer, both of which are combinedto form the electrically conductive material layer. That is, theelectrically conductive material layer can be formed of two (or more)conductive layers having different compositions; or, may be formed of alayer having a single composition. In cases where other metal elementsare present in the layer, gold may be the major component of the layer(e.g., greater than 50% by weight, greater than 75% by weight, orgreater the 90% by weight). Other suitable metal elements includetitanium, tin, nickel, aluminum, and copper, amongst others. Any alloycombinations of these metals (including gold) may be suitable. Incertain embodiments, the layer may consist essentially of gold, meaningthat other elements are present (if at all) in amounts that do notmaterially effect the properties of the layer. As described furtherbelow, it may be advantageous for the electrically conductive materiallayer to be formed of a similar composition as that of one or moreelectrical contacts (e.g., topside contact 28, backside contact 30) towhich the electrically conductive material layer is connected.

The electrically conductive material layer is generally sufficientlythick to ensure suitable conduction. For example, the electricallyconductive material layer may have a thickness between about 100 nm andabout 10 micron, though it should be understood that other thicknessesare also possible. Different portions of the layer may have differentthicknesses.

In the embodiment of FIG. 1, the electrically conductive material layeris formed on sidewalls of the via (with the barrier material layerfunctioning as an intervening layer between the electrically conductivematerial layer and the sidewall, as described further below). Theelectrically conductive material layer may be deposited using knowntechniques including sputtering, evaporative and plating techniques. Asshown, the electrically conductive material layer covers substantiallythe entire surface area of the via sidewalls. In other embodiments, theelectrically conductive material layer may cover only a portion of thesurface area of the via sidewalls (e.g., greater than 50 percent orgreater than 75 percent of the sidewall surface area). In someembodiments, the electrically conductive material layer further extendsfrom the via sidewall onto other portions of the structure. For example,the electrically conductive material layer may extend onto topside 29and/or backside 31. In embodiments in which the electrically conductivematerial layer extends on to topside 29 and/or backside 31, theelectrically conductive material may also form topside contact 28 and/orbackside contact 30.

It should be understood that the electrically conductive material layerneed not be formed in a via and that certain structures of the inventiondo not include a via. For example, the electrically conductive materialmay be formed as a layer on another layer.

In FIG. 1, barrier material layer 22 is positioned so as to separate theelectrically conductive material from certain other regions of thestructure. The barrier material layer generally is formed of materialsuitable for preventing undesired reactions (e.g., chemical reactions)between the electrically conductive material and other regions of thestructure. For example, the barrier material may prevent undesiredchemical reaction between the electrically conductive material and thesubstrate, particularly when the electrically conductive material isformed of gold and the substrate is a silicon substrate. It should alsobe understood that the barrier material may also provide other functionssuch as electrically isolating the electrically conductive material fromother regions of the structure. When providing electrical isolation, thebarrier material is suitably insulating. In other cases, the barriermaterial may also be formed of an electrically conductive material and,at least part of, the barrier material layer may form part of aconductive pathway with the electrically conductive material layer 24.

Suitable compositions for the barrier layer may include one or more ofthe following metals titanium, tungsten, nickel and platinum. When thebarrier layer comprises more than one metal, the metals may be alloyedto form a single composition; or, may be formed as a series of layershaving different compositions which combine to form the barrier layer.In some embodiments, it may be preferred that the barrier layercomprises a titanium-tungsten alloy. When electrically insulating, thebarrier material may be formed of silicon oxide or silicon nitride,amongst other insulators.

In the illustrative embodiment, the barrier material layer is formedacross the entire area between the substrate and the electricallyconductive material layer. That is, the barrier material layer separatesthe substrate (e.g., silicon substrate) and the electrically conductivematerial layer at all locations with the barrier material layer beingdirectly on the sidewalls of the via and the electrically conductivematerial layer being directly on the barrier material layer. Thisarrangement may be preferred to prevent/limit interaction between thesubstrate and the electrically conductive material.

In FIG. 1, the barrier material layer also separates the electricallyconductive material layer from other regions of the structure includinggallium nitride material region 18, as well as strain-absorbing layer12, intermediate layer 15, and transition layer 16. However, it shouldbe understood that in other embodiments the barrier material layer maynot separate the electrically conductive material layer from certainregions of the structure including gallium nitride material region, thestrain-absorbing layer, intermediate layer, or transitionlayer—particularly, if undesired reactions do not take place between theelectrically conductive material layer and such regions.

The structure of FIG. 1 includes a topside contact 28 and a backsidecontact 30. JO A contact includes any suitable conducting structure on asemiconductor device that is designed to be electrically connected to apower source. “Contacts” may also be referred to as electrical contacts,electrodes, terminals, contact pads, contact areas, contact regions andthe like. In some embodiments, contacts may be directly connected to apower source, for example, by wire bonding or air bridging. In otherembodiments, a contact may be indirectly connected to a power source,for example, by a conductive pathway which connects to another contactand then to the power source.

Contacts are formed of conducting materials including certain metals.Any suitable conducting material known in the art may be used. In someembodiments, it may be preferred for at least one of the contacts (orboth contacts in structures having two contacts as in FIG. 1) to beformed of the same material as the electrically conductive materiallayer 24. For example, it may be preferable for the contact(s) tocomprise gold. Gold may be the major component of the layer (e.g.,greater than 50% by weight, greater than 75% by weight, or greater the90% by weight). In certain embodiments, the layer may consistessentially of gold, meaning that other elements are present (if at all)in amounts that do not materially effect the properties of the layer.

The contacts may also be formed of other suitable metals includingtitanium, nickel, aluminum, and copper, amongst others. Any alloycombinations of these metals (including gold) may be suitable. In someembodiments, the composition of the contact may depend upon the type ofmaterial that the contact is formed on. Suitable metals for n-typecontacts include titanium, nickel, aluminum, gold, copper, and alloysthereof. Suitable metals for p-type contacts include nickel, gold, andtitanium, and alloys thereof.

The contacts generally have a thickness sufficient to ensure that thecontact is electrically conductive across its entire physical area.Suitable thicknesses for contacts, for example, are between about 0.05microns and about 10 microns.

In some embodiments, structures of the invention may include more thanone topside contact and/or more than one or backside contact. In somecases, when backside contacts are present, the structures include notopside contact; and, in some cases, when topside contacts are present,the structures include no backside contacts.

Via 20 can enable formation of a conductive pathway within the structurewhen electrically conductive material is deposited therein. As shown,the via can connect two or more contacts on various sides of thestructure which can lead to greater flexibility in device design,amongst other advantages. However, it should also be understood that notall structures of the invention have a via formed therein.

The via may be formed in any suitable shape. The cross-sectional area ofthe via is generally sufficient to enable formation of the desiredstructure (e.g., electrically conductive material layer, barriermaterial layer) therein. The cross-sectional profile of the via may besquare, rectangular, spherical, triangular, or the like. The via mayhave the same cross-sectional profile throughout the via, or may have across-sectional area which changes (e.g., increases, decreases) at anypoint along the depths of the via. For example, in one embodiment, thevia may have a cross-sectional profile which progressively narrows asthe depth of the via increases. In some embodiments, a first portion ofthe via has a relatively large cross-sectional area and a second portionof the via has a smaller cross-sectional area. The first portion mayextend from a topside of the structure and is formed in a first etchingstep; and, the second portion may extend from a backside of thestructure and is formed of a second etching step.

The via may extend from different sides of the structure and to avariety of depths in the structure based on the application. The via mayextend from the topside 29 and/or backside 31. In some cases, the viaextends through the entire structure (e.g., as shown in FIG. 1) and,thus, from both the topside and backside. However, it is also possiblefor the via to extend from only one of the topside or backside andthrough only a portion of the structure. For example, the via may extendfrom the topside to the silicon substrate (particularly, if the siliconsubstrate is sufficiently conductive and may be grounded). Also, the viamay extend from the backside through the silicon substrate to a pointwithin the gallium nitride material region. Other via arrangements arealso possible.

In the illustrative embodiment, a portion of the via remains unfilledwith material. However, in other cases, the via may be completely filledwith material. Also, it should be understood that other layers (inaddition to the barrier material layer and the electrically conductivematerial layer) may be formed in the via.

It should be understood that the electrically conductive material layersand/or the barrier material layers described above may be formed withina portion, or the entire via, in any via arrangement.

In certain preferred embodiments, substrate 14 is a silicon substrate.As used herein, a silicon substrate refers to any substrate thatincludes a silicon surface. Examples of suitable silicon substratesinclude substrates that are composed entirely of silicon (e.g., bulksilicon wafers), silicon-on-insulator (SOI) substrates,silicon-on-sapphire substrate (SOS), and SIMOX substrates, amongstothers. Suitable silicon substrates also include substrates that have asilicon wafer bonded to another material such as diamond, AlN, or otherpolycrystalline materials. Silicon substrates having differentcrystallographic orientations may be used, though single crystal siliconsubstrates are preferred. In some cases, silicon (111) substrates arepreferred. In other cases, silicon (100) substrates are preferred. Insome embodiments, silicon substrates having a relatively highresistivity are preferred. For example, in some cases, the siliconsubstrate has a resistivity of greater than 10 kilo-Ohms.

It should be understood that other types of substrates may also be usedincluding sapphire, silicon carbide, indium phosphide, silicongermanium, gallium arsenide, gallium nitride, aluminum nitride, or otherIII-V compound substrates. However, in embodiments that do not usesilicon substrates, all of the advantages associated with siliconsubstrates may not be achieved. In some embodiments, it may bepreferable to use non-nitride material-based substrates such as silicon,sapphire, silicon carbide, indium phosphide, silicon germanium andgallium arsenide.

Substrate 14 may have any suitable dimensions. Suitable diametersinclude, but are not limited to, about 2 inches (or 50 mm), 4 inches (or100 mm), 6 inches (or 150 mm), and 8 inches (or 200 mm). In someembodiments, it may be preferable to use a silicon substrate havingrelatively large diameters of at least about 4 inches (or 100 mm) or atleast about 6 inches (or 150 mm). As described further below, thearrangement of layers between the silicon substrate and the galliumnitride material region (e.g., strain-absorbing layer 12, intermediatelayer 15, and transition layer 16) may be designed to enable highquality gallium nitride material to be deposited even at relativelylarge diameters. In some cases, it may be preferable for the substrateto be relatively thick, such as greater than about 125 micron (e.g.,between about 125 micron and about 800 micron, or between about 400micron and 800 micron). Relatively thick substrates may be easy toobtain, process, and can resist bending which can occur, in some cases,in thinner substrates. In other embodiments, thinner substrates (e.g.,less than 125 microns) are used, though these embodiments may not havethe advantages associated with thicker substrates, but can have otheradvantages including facilitating processing and/or reducing the numberof processing steps. In some processes, the substrate initially isrelatively thick (e.g., between about 200 microns and 800 microns) andthen is thinned during a later processing step (e.g., to less than 150microns).

In some preferred embodiments, the substrate is substantially planar inthe final device or structure. Substantially planar substrates may bedistinguished from substrates that are textured and/or have trenchesformed therein (e.g., as in U.S. Pat. No. 6,265,289). As shown, thelayers/regions of the device (e.g., strain-absorbing layer, intermediatelayer, transition layer, gallium nitride material region) may also besubstantially planar in the final device or structure. As describedfurther below, such layers/regions may be grown in vertical (e.g.,non-lateral) growth processes. Planar substrates and layers/regions canbe advantageous in some embodiments, for example, to simplifyprocessing. Though it should be understood that, in some embodiments ofthe invention, lateral growth processes may be used, as describedfurther below, which may use textured substrates.

Strain-absorbing layer 12 helps absorb strain that arises due to latticedifferences between the crystal structure of the substrate and thecrystal structure of overlying layers (e.g., intermediate layer 15,transition layer 16, gallium nitride material region 18). In the absenceof the strain-absorbing layer, this strain is typically accommodated bythe generation of misfit dislocations in the layer that forms aninterface with the substrate. Thus, by providing an alternativemechanism for accommodating stress, the presence of the strain-absorbinglayer may reduce the generation of misfit dislocations. Suitablestrain-absorbing layers have been described, for example, incommonly-owned, co-pending U.S. patent application Ser. No. 10/879,703,filed Jun. 28, 2004, which is incorporated herein by reference.

Furthermore, the strain-absorbing layer can help absorb strain thatarises due to differences in the thermal expansion rate of the substrateas compared to the thermal expansion rate of overlying layer(s)including the gallium nitride material region, Such differences can leadto formation of defects (e.g., misfit dislocations) at the overlyinglayer/substrate interface, or cracking in overlying layer(s) includingthe gallium nitride material region. As described further below,transition layer 16 also helps absorb this thermally-induced strain.

In certain preferred embodiments, strain-absorbing layer 12 is formed ofa silicon nitride-based material. Silicon nitride-based materialsinclude any silicon nitride-based compound (e.g., Si_(x)N_(y), such asSiN and Si₃N₄, SiCN, amongst others) including non-stoichiometricsilicon nitride-based compounds. In some embodiments, a SiNstrain-absorbing layer may be preferred. Silicon nitride material-basedstrain-absorbing layers may be particularly preferred when formeddirectly on a silicon substrate, as described further below.

It should also be understood that it is possible for thestrain-absorbing layer to be formed of other types of materialsaccording to other embodiments of the invention. Though all of theadvantages associated with silicon nitride-based materials may not beachieved in these embodiments.

In some embodiments, it is preferable for the strain-absorbing layer tohave an amorphous (i.e., non-crystalline) crystal structure. Amorphousstrain-absorbing layers are particularly effective in accommodatingstrain and, thus, reducing the generation of misfit dislocations andother types of defects.

However, it should be understood that in certain embodiments of theinvention the strain-absorbing layer may have a single crystal orpoly-crystalline structure. In these cases, however, all of theadvantages associated with the amorphous layer may not be realized.

In some embodiments, it is preferred for the strain-absorbing layer tobe very thin, particularly when formed of amorphous and/or siliconnitride-based materials. It has been discovered that very thinstrain-absorbing layers (e.g., silicon nitride-based strain-absorbinglayers) may enable formation of overlying layer(s) having an epitaxialrelationship with the substrate, while also being effective in reducingthe number of misfit dislocations. In certain cases (e.g., when thestrain-absorbing layer is amorphous), it is desirable for thestrain-absorbing layer to have a thickness that is large enough toaccommodate sufficient strain associated with lattice and thermalexpansion differences between the substrate and overlying layer(s) toreduce generation of misfit dislocations. In these cases, it may also bedesirable for the strain-absorbing layer to be thin enough so that theoverlying layer(s) have an epitaxial relationship with the substrate.This can be advantageous for formation of a high quality, single crystalgallium nitride material region. If the strain-absorbing layer is toothick, then the overlying layer(s) may not be formed with epitaxialrelationship with the substrate.

In some embodiments, the strain-absorbing layer has a thickness of lessthan about 100 Angstroms which, in these embodiments, can allow theepitaxial relationship between the substrate and the overlying layer. Insome embodiments, it may be preferable for the strain-absorbing layer tohave a thickness of less than about 50 Angstroms to allow for theepitaxial relationship.

The strain-absorbing layer may have a thickness of greater than about 10Angstroms which, in these embodiments, is sufficient to accommodatestrain (e.g., resulting from lattice and thermal expansion differences)and can facilitate forming a strain-absorbing layer that covers theentire substrate, as described further below. In other embodiments, thestrain-absorbing layer may have a thickness of greater than about 20Angstroms to sufficiently accommodate strain. Suitable thickness rangesfor the strain-absorbing layer include all of those defined by theranges described above (e.g., greater than about 10 Angstroms and lessthan about 100 Angstroms, greater than about 10 Angstroms and less thanabout 50 Angstroms, and the like), Also, the strain-absorbing layerthickness may be between about 20 Angstroms and about 70 Angstroms.

It should be understood that suitable thicknesses of thestrain-absorbing layer may depend on a number of factors including thecomposition and crystal structure of the strain-absorbing layer; thecomposition, thickness and crystal structure of the overlying layer; aswell as the composition, thickness, and crystal structure of thesubstrate, amongst other factors. Suitable thicknesses may be determinedby measuring the effect of thickness on misfit dislocation density andother factors (e.g., the ability to deposit an overlying layer having anepitaxial relationship with the substrate, etc.). It is also possiblefor the strain-absorbing layer to have a thickness outside the aboveranges.

As described further below, in some embodiments, the strain-absorbinglayer may be formed by nitridating a top surface region of a siliconsubstrate. That is, the surface region of the substrate may be convertedfrom silicon to a silicon nitride-based material to form thestrain-absorbing layer. It should be understood that, as used herein,such strain-absorbing layers may be referred to as being “formed on thesubstrate”, “formed over the substrate”, “formed directly on thesubstrate” and as “covering the substrate”. Such phrases also refer tostrain-absorbing layers that are formed by depositing a separate layer(e.g., using a separate nitrogen source and silicon source) on the topsurface of the substrate and are not formed by converting a surfaceregion of the substrate.

In the illustrative embodiment, the strain-absorbing layer coverssubstantially the entire top surface of the substrate. This arrangementmay be preferable to minimize the number of defects (e.g., misfitdislocations) in the overlying layer(s). In other embodiments, thestrain-absorbing layer may cover a majority of the top surface of thesubstrate (e.g., greater than 50 percent or greater than 75 percent ofthe top surface area).

The extent that the strain-absorbing layer covers the substrate (and thearea between the overlying layer and the substrate) in the presentinvention may be is distinguished from certain prior art techniques inwhich a discontinuous silicon nitride layer is formed (in some cases,inadvertently) between a silicon substrate and an overlying layer.

It should be understood that, in other embodiments, the strain-absorbinglayer may be positioned elsewhere in the structure including between twodifferent layers. In some cases, structures of the invention do notinclude an intermediate layer.

The structure of FIG. 1 includes an intermediate layer 15 formed of anitride-based material that overlies the strain-absorbing layer.Suitable nitride-based materials include, but are not limited to,aluminum nitride materials (e.g., aluminum nitride, aluminum nitridealloys) and gallium nitride materials (e.g., gallium nitride, galliumnitride alloys). In some cases, the intermediate layer has a constantcomposition.

It may be preferable for the intermediate layer to have a single crystalstructure. As noted above, in some embodiments, the thickness of thestrain-absorbing layer is controlled so that the intermediate layer hasan epitaxial relationship with the substrate. It may be advantageous forthe intermediate layer to have a single crystal structure because itfacilitates formation of a single crystal, high quality gallium nitridematerial region. In some embodiments, the intermediate layer has adifferent crystal structure than the substrate.

It should also be understood that the intermediate layer may not have asingle crystal structure and may be amorphous or polycrystalline, thoughall of the advantages associated with the single crystal intermediatelayers may not be achieved.

The intermediate layer may have any suitable thickness. For example, theintermediate layer may be between about 10 nanometers and 5 microns,though other thicknesses are also possible.

It should also be understood that not all structures of the inventioninclude an intermediate layer.

In the illustrative embodiment, transition layer 16 is formed directlyon the intermediate layer. In certain embodiments, such as when theintermediate layer has a constant composition, it may be preferred forthe transition layer to be formed of a compositionally-graded material(e.g., a compositionally-graded nitride-based material). Suitablecompositionally-graded layers have been described in commonly-owned U.S.Pat. No. 6,649,287 which is incorporated by reference above.Compositionally-graded transition layers have a composition that isvaried across at least a portion of the layer. Compositionally-gradedtransition layers are particularly effective in reducing crack formationin gallium nitride material regions formed on the transition layer bylowering thermal stresses that result from differences in thermalexpansion rates between the gallium nitride material and the substrate(e.g., According to one set of embodiments, the transition layer iscompositionally-graded and formed of an alloy of gallium nitride such asAl_(x)In_(y)Ga_((1-x-y))N, Al_(x)Ga_((1-x))N, and In_(y)Ga_((1-y))N. Inthese embodiments, the concentration of at least one of the elements(e.g., Ga, Al, In) of the alloy is varied across at least a portion ofthe thickness of the transition layer. When transition layer 16 has anAl_(x)In_(y)Ga_((1-x-y))N composition, x and/or y may be varied. Whenthe transition layer has a Al_(x)Ga_((1-x))N composition, x may bevaried. When the transition layer has a In_(y)Ga_((1-y))N composition, ymay be varied.

In certain preferred embodiments, it is desirable for the transitionlayer to have a low gallium concentration at a back surface which isgraded to a high gallium concentration at a front surface. It has beenfound that such transition layers are particularly effective inrelieving internal stresses within gallium nitride material region 18.For example, the transition layer may have a composition ofAl_(x)Ga_((1-x))N, where x is decreased from the back surface to thefront surface of the transition layer (e.g., x is decreased from a valueof 1 at the back surface of the transition layer to a value of 0 at thefront surface of the transition layer).

In one preferred embodiment, structure 10 includes an aluminum nitrideintermediate layer 15 and a compositionally-graded transition layer 16.The compositionally-graded transition layer may have a composition ofAl_(x)Ga_((1-x))N, where x is graded from a value of 1 at the backsurface of the transition layer to a value of 0 at the front surface ofthe transition layer. The composition of the transition layer, forexample, may be graded discontinuously (e.g., step-wise) orcontinuously. One discontinuous grade may include steps of AlN,Al_(0.6)Ga_(0.4)N and Al_(0.3)Ga_(0.7)N proceeding in a direction towardthe gallium nitride material region.

In some embodiments, the compositionally-graded transition layercomprises a superlattice which, for example, includes alternating layersof nitride-based materials (e.g., gallium nitride material and aluminumnitride material)

It should be understood that, in other cases, transition layer 16 mayhave a constant composition and may not be compositionally-graded.

The strain-absorbing layer, intermediate layer and transition layer aretypically not part of the active region of the device. As describedabove, these layers may be formed to facilitate deposition of galliumnitride material region 18. However, in some cases, the intermediatelayer and/or transition layer may have other functions includingfunctioning as a heat spreading layer that helps remove heat from activeregions of the semiconductor structure during operation of a device. Forexample, such transition layers that function as heat spreading layershave been described in commonly-owned U.S. Pat. No. 6,956,250 which isincorporated herein by reference and is based on U.S. patent applicationSer. No. 09/792,409 entitled “Gallium Nitride Materials IncludingThermally-Conductive Regions,” filed Feb. 23, 2001.

Active regions of the device may be formed in gallium nitride materialregion 18. Gallium nitride material region 18 comprises at least onegallium nitride material layer. As used herein, the phrase “galliumnitride material” refers to gallium nitride (GaN) and any of its alloys,such as aluminum gallium nitride (Al_(x)Ga_((1-x))N), indium galliumnitride (In_(y)Ga_((1-y))N), aluminum indium gallium nitride(Al_(x)In_(y)Ga_((1-x-y))N), gallium arsenide phosporide nitride(GaAs_(a)P_(b)N_((1-a-b))), aluminum indium gallium arsenide phosporidenitride (Al_(x)In_(y)Ga_((1-x-y))As_(a)P_(b) N_((1-a-b))), amongstothers. Typically, when present, arsenic and/or phosphorous are at lowconcentrations (i.e., less than 5 weight percent). In certain preferredembodiments, the gallium nitride material has a high concentration ofgallium and includes little or no amounts of aluminum and/or indium. Inhigh gallium concentration embodiments, the sum of (x+y) may be lessthan 0.4, less than 0.2, less than 0.1, or even less. In some cases, itis preferable for the gallium nitride material layer to have acomposition of GaN (i.e., x+y=0). Gallium nitride materials may be dopedn-type or p-type, or may be intrinsic. Suitable gallium nitridematerials have been described in U.S. Pat. No. 6,649,287, incorporatedby reference above.

In some cases, the gallium nitride material region includes only onegallium nitride material layer. In other cases, the gallium nitridematerial region includes more than one gallium nitride material layer.For example, the gallium nitride material region may include multiplelayers. In certain embodiments, it may be preferable for the galliumnitride material of a first layer to have an aluminum concentration thatis greater than the aluminum concentration of the gallium nitridematerial of a second layer of region 18. For example, the value of x inthe gallium nitride material of first layer (with reference to any ofthe gallium nitride materials described above) may have a value that isbetween 0.05 and 1.0 greater than the value of x in the gallium nitridematerial of second layer, or between 0.05 and 0.5 greater than the valueof x in the gallium nitride material of the first layer. For example,the second layer may be formed of Al_(0.26)Ga_(0.74)N, while the firstlayer is formed of GaN. This difference in aluminum concentration maylead to formation of a highly conductive region at the interface of thelayers (i.e., a 2-D electron gas region). A third layer, for example,may be formed of GaN.

The general semiconductor structure illustrated in FIG. 1 may beincorporated into a variety of semiconductor devices including devicesdescribed in commonly-owned U.S. Pat. No. 7,071,498 which isincorporated herein by reference and is based on commonly-owned,co-pending U.S. patent application Ser. No. 10/740,376, filed on Dec.17, 2003, and entitled “Gallium Nitride Material Devices Including anElectrode-Defining Layer and Methods of Forming the Same”. Suitabledevices include, but are not limited to, electronic devices includingtransistors (e.g., FETs), SAW devices, and sensors; as well as,light-emitting devices including LEDs and laser diodes. The devices haveactive regions that are typically, at least in part, within the galliumnitride material region. Also, the devices include a variety of otherfunctional layers and/or features (e.g., electrodes).

FIG. 2 illustrates a semiconductor device 50 according to an embodimentof the invention. In this illustrative embodiment, the semiconductordevice is a transistor (e.g., a field effect transistor). The deviceincludes electrically conductive material 24 that extends within via 20to form at least part of an electrically conductive pathway thatconnects backside contact 30 to topside contact 28. As shown, theelectrically conductive material is formed of the same composition (and,in some cases, in the same processing step) as the backside contact. Forexample, the electrically conductive material and the backside contactmay be a gold-based composition as described above. However, it shouldbe understood that the invention is not limited in this regard and thatthe electrically conductive material may be formed from a differentmaterial than the backside contact.

Barrier material layer 22 separates the electrically conductive material24 from substrate 14 (e.g., a silicon substrate). In the embodiment ofFIG. 2, barrier material layer 22 also forms part of the conductivepathway that connects backside contact 30 to topside contact 28. Thus,in this case, the barrier material layer is formed of a conductivematerial as described above. In the illustrative embodiment, layer 52may also form part of the electrically conductive pathway that connectsthe backside contact to the topside contact. An encapsulating layer 54may electrically isolate layer 52 from other regions of the deviceincluding gallium nitride material layer 18 and transition layer 16. Theencapsulating layer may be formed of an insulating material such as anoxide (e.g., silicon oxide) or a nitride (e.g., silicon nitride).

Device 50 includes a source electrode 56, a gate electrode 58 and adrain electrode 60. In this embodiment, the source electrode iselectrically connected to the topside contact and to the backsidecontact through a conductive pathway that includes the electricallyconductive material 24.

The gate electrode may be defined, in part, in a via formed in anelectrode-defining layer 62. The electrode-defining layer may be apassivating layer and may be formed of a silicon nitride-based material(e.g., Si₃N₄). Suitable electrode-defining layers and source-gate-drainarrangements have been described in U.S. Pat. No. 7,071,498 which isincorporated by reference above.

The device may also include a source field plate 66 formed, in part, onthe encapsulation layer and is electrically connected to the sourceelectrode. In the illustrative embodiment, the source field plateextends in a direction toward the gate electrode. Suitable source fieldplates have been described in commonly-owned, co-pending U.S. patentapplication Ser. No. not yet assigned, filed on Nov. 30, 2006, andentitled “Gallium Nitride Material Devices and Associated Methods” whichis incorporated herein by reference.

The device may also include a metal layer 64 which may connect otherelectrically conductive regions on the device to one another.

In the illustrative embodiment, strain absorbing layer 12 andintermediate layer 15 are not shown, though it should be understood thatthese layers may also be present. Other variations to the device shownin FIG. 1 are also possible and would be understood by one of ordinaryskill in the art.

In some embodiments, the device may be attached to a component such as apackage. For example, the device may be bonded to the back surface ofthe device with a eutectic comprising an electrically conductivematerial (e.g., gold) and silicon. The bonding process may involveheating the structure to form a liquid eutectic comprising theelectrically conductive material and silicon, and cooling the liquideutectic to form a bond between the semiconductor structure and thecomponent.

Semiconductor structures of the invention may be manufactured usingknown semiconductor processing techniques. It should be understood thatvariations to this process are within the scope of the presentinvention.

The substrates (e.g., silicon) used in accordance with the invention aregenerally commercially available. In embodiments in which astrain-absorbing layer is a silicon nitride-based material (e.g.,amorphous SiN), the strain-absorbing layer may be formed by nitridatinga top surface of a silicon substrate as noted above. In a nitridationprocess, nitrogen reacts with a top surface region of the siliconsubstrate to form a silicon nitride-based layer. The top surface may benitridated by exposing the silicon substrate to a gaseous source ofnitrogen at elevated temperatures. For example, ammonia may beintroduced into a reaction chamber in which a silicon substrate ispositioned. The temperature in the reaction chamber may be between about1000° C. and about 1100° C. and the pressure may be between about 20torr and about 40 torr (in some cases, about 30 torr. The reactionbetween nitrogen and the silicon substrate is allowed to proceed for areaction time selected to produce a layer having a desired thickness.

It should be understood that other processes may be used to form siliconnitride-based strain-absorbing layers including processes (e.g., CVDprocesses) that use separate nitrogen and silicon sources. Also, whenthe strain-absorbing layer is formed of another type of material(non-silicon nitride-based material), other deposition processes knownin the art are used.

In some embodiments, the strain-absorbing layer may be formed in-situwith certain overlying layers (e.g., the intermediate layer, galliumnitride material region). That is, the strain-absorbing layer may beformed during the same deposition step as the intermediate layer (and,in some cases, subsequent layers). In processes that grow a siliconnitride-based material strain-absorbing layer by introducing a nitrogensource (e.g., ammonia) into a reaction chamber as described above, asecond source gas may be introduced into the chamber after a selectedtime delay after the nitrogen source. The second source reacts with thenitrogen source to form the overlying layer (e.g., intermediate layer),thus, ending growth of the strain-absorbing layer. For example, when theoverlying layer (e.g., intermediate layer) is formed of aluminumnitride, an aluminum source (e.g., trimethylaluminum) is introduced intothe chamber at a selected time after the nitrogen source (e.g.,ammonia). The time delay is selected so that the strain-absorbing layergrows to a desired thickness. The reaction between the second source(e.g., aluminum source) and the nitrogen source is allowed to proceedfor a sufficient time to produce the intermediate layer. When theoverlying layer (e.g., intermediate layer) has a single crystalstructure, the reaction conditions are selected appropriately. Forexample, the reaction temperature may be greater than 700° C., such asbetween about 1000° C. and about 1100° C. In some cases, lower growthtemperatures may be used including temperatures between about 500° C.and about 600° C.

It should also be understood that the strain-absorbing layer may beformed in a separate process than the intermediate layer and subsequentlayers. For example, the strain-absorbing layer may be formed on thesubstrate in a first process. Then, at a later time, the intermediatelayers may be formed on the strain-absorbing layer in a second process.

Transition layer 16 and gallium nitride material region 18 may also begrown in the same deposition step as the intermediate layer and thestrain-absorbing layer. In such processes, suitable sources areintroduced into the reaction chamber at appropriate times. SuitableMOCVD processes to form compositionally-graded transition layers andgallium nitride material region over a silicon substrate have beendescribed in U.S. Pat. No. 6,649,287 incorporated by reference above.When the gallium nitride material region has different layers, in somecases, it is preferable to use a single deposition step to form theentire region. When using the single deposition step, the processingparameters may be suitably changed at the appropriate time to form thedifferent layers.

It should also be understood that the transition layer and the galliumnitride material region may be grown separately from thestrain-absorbing layer and intermediate layer. The gallium nitridematerial region and transition layer may be grown in a vertical growthprocess. That is, these regions are grown in a vertical direction withrespect to underlying layers. The ability to vertically grow the galliumnitride material region having defect densities may be advantageous ascompared to lateral growth processes which may be more complicated.

Vias in the devices and structures may be formed using conventionaltechniques. For example, etching techniques may be used to form vias. Inembodiments that include vias that extend through structures, a firstportion of the via may be formed in a first etching step and a secondportion in a second step.

The conductive and/or metal layers may be formed using suitableconventional techniques including sputtering, electroplating andevaporative techniques. In some embodiments, it may be preferred thatthe barrier material layers are sputtered. In some embodiments, it maybe preferred that the conductive material layer is electroplated (e.g.,when the conductive material layer comprises a gold composition asdescribed above). The backside contact and/or the topside contact mayalso be electroplated (e.g., when the conductive material layercomprises a gold composition as described above). In cases when thebackside contact and the conductive material layer are formed of thesame composition, it may be preferable that they are formed in the sameprocessing step (e.g., electroplating).

It should also be understood that other processes may be used to formstructures and devices of the present invention as known to those ofordinary skill in the art.

Having thus described several aspects of at least one embodiment of thisinvention, it is to be appreciated various alterations, modifications,and improvements will readily occur to those skilled in the art. Suchalterations, modifications, and improvements are intended to be part ofthis disclosure, and are intended to be within the spirit and scope ofthe invention. Accordingly, the foregoing description and drawings areby way of example only.

1-57. (canceled)
 58. A transistor comprising a source electrode, a drainelectrode and a gate electrode, said transistor further comprising: asubstrate; a transition layer situated over said substrate; a galliumnitride layer situated over said transition layer; a via that extendsthrough at least a portion of said substrate; an electrically conductivelayer on a back surface of said substrate; said via coupling saidelectrically conductive layer on said back surface of said substrate toa top-side contact formed over said substrate, said top-side contactoverlying a conductive field plate.
 59. The transistor of claim 58,wherein said substrate comprises silicon.
 60. The transistor of claim58, wherein said electrically conductive layer comprises aluminum. 61.The transistor of claim 58, wherein said electrically conductive layerincludes a first portion comprising gold and a second portion comprisingaluminum.
 62. The transistor of claim 58 further comprising apassivating layer situated over said gallium nitride layer.
 63. Thetransistor of claim 58, wherein said gate electrode is defined by anelectrode-defining layer comprising silicon nitride.
 64. The transistorof claim 58, wherein said transition layer is compositionally-graded.65. The transistor of claim 58 further comprising a silicon nitridelayer between said substrate and said transition layer.
 66. A transistorcomprising a source electrode, a drain electrode and a gate electrode,said transistor further comprising: a substrate; a silicon nitride layersituated over said substrate; a transition layer situated over saidsilicon nitride layer; a gallium nitride layer situated over saidtransition layer; a via that extends through at least a portion of saidsubstrate; an electrically conductive layer on a back surface of saidsubstrate; said via coupling said electrically conductive layer on saidback surface of said substrate to a top-side contact formed over saidsubstrate.
 67. The transistor of claim 66, wherein said substratecomprises silicon.
 68. The transistor of claim 66, wherein saidelectrically conductive layer comprises aluminum.
 69. The transistor ofclaim 66, wherein said electrically conductive layer includes a firstportion comprising gold and a second portion comprising aluminum. 70.The transistor of claim 66 further comprising a passivating layersituated over said gallium nitride layer.
 71. The transistor of claim66, wherein said gate electrode is defined by an electrode-defininglayer comprising silicon nitride.
 72. The transistor of claim 66,wherein said transition layer is compositionally-graded.